Conventionally, a slave cylinder of a clutch release mechanism is actuated by hydraulic pressure produced by a master cylinder connected to a clutch pedal. The slave cylinder in turn axially displaces a release bearing via a release fork to cause the release bearing to either push or pull a central part of a diaphragm spring of the clutch device, and a pressure plate is displaced away from a clutch disk.
With the increase in the maximum output of engines, there has been a strong demand for an ever greater capacity of the clutch device to allow the pressure of the pressure plate to be increased. As a result, the force required for releasing the clutch is increased, and it has become a major task to reduce the release force in designing a clutch release mechanism.
It is possible to reduce the force required for releasing the clutch by increasing the bore size of the slave cylinder, to use a booster making use of the negative pressure of the engine intake system, and to increase the lever length of the release fork. However, these measures require the overall size of the clutch device to be increased, and are not acceptable in all of the applications.
An alternative approach to solving this problem is the use of an annular slave cylinder disposed coaxially around an input shaft of a transmission device, and adopted to directly actuate a release bearing. A release cylinder device using such an annular cylinder, which is also known by the name of concentric slave cylinder (CSC) type release cylinder device, is proposed, for instance, in the U.S. Pat. No. 4,938,332. According to this proposal, a tubular piston is received in an annular recess in an axially slidable manner, and the hydraulic pressure is supplied to a bottom end of the annular recess. The use of a tubular piston allows the pressure receiving surface of the piston to be maximized for a given size of the clutch release cylinder device.
However, according to the CSC type release cylinder device, the inner surface of the annular recess must to finished at an extremely high precision for the tubular piston to be received in the annular cylinder in a liquid tight manner. Furthermore, a bottom end of the annular recess is provided with an inlet for hydraulic fluid and an air purge hole. Due to these factors, the annular cylinder is known to be expensive to manufacture.